Muscle Growth Part 1: The Science Behind Why, And How, Does A Muscle Grow And Get Stronger?

In concept, weight training is a very simple practice. You lift weights, you wait a while, you do it again. You improve over time and eventually you are stronger and bigger than you were before. When you strip it down it’s really quite simple isn’t it? The problem is things don’t always go as smoothly as the above description would imply.

The Size And Strength Relationship

In bodybuilding circles there is the common misconception that muscle mass increases and strength increases are not necessarily related. That is to say, that you can increase the size of a muscle without it getting stronger. This mistaken belief presents itself commonly in the old “Bodybuilders aren’t as strong as Powerlifters” argument. If strength was related to muscle mass, wouldn’t Powerlifters be bigger than Bodybuilders?

The explanation is simple: Strong people usually have better mechanical advantages than weaker people.

This includes more favorable joint lengths and connective tissue factors (including attachment placings and superior tendon and ligament strength). They may have more type II fibers than others and/or a more efficient nervous system (which can be trained for). A muscle can be trained to get stronger but not bigger – this depends on rep range, training volume and frequency. However, if a muscle gets larger it must also get stronger in the rep range over which it was trained. Likewise, if a muscle gets stronger in a rep range conducive to producing growth then the muscle will also get larger.

It is a scientifically verified physiological fact that muscle size and strength are directly related. Let’s take a look at what happens to a muscle when you train it.

Segment from the Neuromuscular System series:

“Muscle biopsies of experienced bodybuilders have shown that it was the size of the individual fibers within their muscles that was responsible for the abnormal muscle size and not the actual number of fibers present.” Although there is some evidence that extreme conditions may result in modest increases in fiber number (hyperplasia), the mechanism responsible for muscle size growth is hypertrophy – the increase in size of existing muscle fibers.

Another segment from the Neuromuscular System series:

“It is also worthy of note that contractile machinery comprises about 80% of muscle fiber volume. The rest of the volume is accounted for by tissue that supplies energy to the muscle or is involved with the neural drive.”

This tells us that there are a couple of ways to increase muscle size.

Increase the volume of the tissue that supplies energy to the muscle or is involved with the neural drive – called sarcoplasmic hypertrophy.

Increase the volume of contractile machinery – called sarcomere hypertrophy.

Let’s take a look at both routes.

Sarcoplasmic Hypertrophy

Increasing the volume of the tissue that supplies energy to the muscle or is involved with the neural drive: Intimately involved in the production of ATP are intracellular bodies called ‘mitochondria’. Muscle fibers will adapt to high volume (and higher rep) training sessions by increasing the number of mitochondria in the cells. They will also increase the concentrations of the enzymes involved in the oxidative phosphorylation and anaerobic glycolysis mechanisms of energy production and increase the volume of sarcoplasmic fluid inside the cell (including glycogen) and also the fluid between the actual cells. This type of hypertrophy produces very little in the way of added limit strength but has profound effects on increasing strength-endurance (the ability to do reps with a certain weight) because it dramatically increases the muscles’ ability to produce ATP. Adaptations of this sort are characteristic of Bodybuilders’ muscles.

It should also be obvious that as the volume of the tissue that supplies energy to the muscle represents only around 20% of the total muscle cell volume in untrained individuals, this isn’t where the majority of growth potential lies.

Hypertrophy Factor

Sarcoplasmic hypertrophy of muscle cells does directly produce moderate increases in size. But also, ATP is the source of energy for all muscular contraction – type II fibers included. Wouldn’t having more of this in the muscle, and having the ability to produce greater intramuscular quantities at any one time, be an asset? The answer is, clearly, “yes”. That’s where a major portion of the importance of sarcoplasmic hypertrophy comes into Bodybuilding. As for increasing the tissue that is involved with the neural drive, this would theoretically occur in response to the need for contracting cells with hypertrophied contractile machinery. Directly, it would produce very little in the way of added size. In addition, there are other intracellular bodies whose growth and/or proliferation would fall under the category of sarcoplasmic hypertrophy.

These would be organelles such as the ribosomes, which are involved in protein synthesis. As in the case of neural drive machinery, in most cases they would increase in size or number only to support sarcomere hypertrophy. They would have little direct impact on overall muscle size.

Sarcomere Hypertrophy

Increasing the volume of contractile machinery: The vast majority of the volume of each muscle cell (~80%) is made up of contractile machinery. Therefore, therein lies the greatest potential for increasing muscle cell size. Trained muscle responds by increasing the number of actin/myosin filaments (sarcomeres) that it contains – this is, primarily, what is responsible for the increased strength and size. But before a muscle will grow like this it has to be ‘broken down’.

Let’s take a look at both the ‘breaking down’ and ‘building up’ processes:

The Process Of Exercise-Induced Muscle Cell Damage

When a muscle fiber develops sufficient tension for sufficient time, increasing fatigue impairs the actin/myosin cross-bridge cycling necessary for the contractile filaments to maintain force production. This impaired cross-bridge cycling under load results in trauma to the contractile filaments as some cross-bridges are subjected to tensions greater than they can structurally support. Additionally, training leads to post-workout breaches in plasma membrane integrity that allow calcium to leak into the muscle cells (there is much more calcium in the blood than in the muscle cells). This intracellular increase in calcium levels activates enzymes called ‘calpains’ which remove pieces of the damaged contractile filaments (called ‘easily releasable myofilaments’).

A protein called ‘ubiquitin’ (which is present in all muscle cells) binds to the removed pieces of filaments thus ‘identifying’ them for destructive purposes. At this time, neutrophils (a type of granular white blood cell) are chemically attracted to the area and rapidly increase in number.

The Breakdown

They release toxins, including oxygen radicals, which increase membrane permeability and phagocytize (ingest and destroy) the tissue debris that the calcium-mediated pathways released. Neutrophils don’t remain around more than a day or two, but are complimented by the appearance of monocytes also attracted to the damaged area. Monocytes (a type of phagocytic cell) enter the damaged muscle and form into macrophages (another phagocytic cell) that also release toxins and phagocytize damaged tissue. Once the phagocytic stage commences, the damaged fibers are rapidly broken down by lysosomal proteases, free O2 radicals, and other substances produced by macrophages. The muscle is now in a weaker state than before it was trained. Incidently, macrophages have an essential role in initiating tissue repair. Unless damaged muscle is invaded by macrophages, activation of satellite cells and muscle repair does not occur. Also, increased intracellular Ca++ concentrations are known to activate an enzyme called phospholipase A2.

This enzyme releases arachidonic acid from the plasma membrane which is then formed into prostaglandins (primarily PGE2) and other eicosanoids that contribute to the degradative processes. So, now that we’ve looked briefly at the process of post-exercise muscle degradation, how does it grow?

The Process Of Exercise-Induced Muscle Growth

Muscle cells have many nuclei and other intracellular organelles. This is because nuclei are intimately involved in the protein synthesis process (don’t forget, actin and myosin are proteins), and a single nuclei can only support the manufacturing of a limited amount of protein. If muscle cells didn’t have multiple nuclei they would be very small muscle cells indeed. So if a muscle is to grow beyond its current size (i.e. synthesize contractile proteins – actin and myosin) it has to increase the number of nuclei that it contains (called the ‘myonuclei number’).

How does it do this?

Around the muscle cells are myogenic stem cells called ‘satellite cells’ (or ‘myoblasts’). Under the right conditions these cells become more ‘like’ muscle cells and actually donate their nuclei to the muscle fibers, thereby increasing myonuclei number. For this to happen, several things need to take place. One, the number of satellite cells has to increase (called ‘proliferation’). Two, they have to become more ‘like’ muscle cells (called ‘differentiation’). And three, they have to fuse with the needy muscle cells.
When the sarcolemma (the muscle cell wall) is ‘damaged’ by tension (as in weight training or even stretching) growth factors are produced and released in the cell. There are several different types of growth factors. The most significant are:

Insulin-like Growth Factor 1 (IGF-1)

Fibroblast Growth Factor (FGF)

Transforming Growth Factor -Beta Superfamily (TGF-beta)

These growth factors can then leave the cell and go out into the surrounding area because sarcolemma permeabilty has been increased due to the ‘damage’ done during contraction. Once outside the muscle cell these growth factors cause the satellite cells to proliferate (mainly FGF does this) and differentiate (mainly IGF-1 does this). TGF-beta’s role is one of mediation – in this case it inhibits growth. After this process the satellite cells then fuse with the muscle cells and donate their nuclei, giving the muscle cells the ‘ability’ to grow.
Now factors that promote protein synthesis such as IGF-1, growth hormone (GH), testosterone and some prostaglandins can commence the growth process. Protein synthesis occurs because a genetically-coded substance called ‘messenger RNA’ (mRNA) is sent out from the nucleus to the ribosomes. The nucleus is believed to release increased mRNA in response to tension and/or myofibrillar damage done as a result of insufficient cycling of actin-myosin cross-bridges during intense muscular contractions, though this mechanism is not fully understood.

The mRNA contains the ‘instructions’ for the ribosomes to synthesize proteins, and so the process of constructing contractile (actin and myosin) and structural proteins (for the other components of the cell) from the amino acids taken into the cell from the bloodstream is set off.

Several substances can influence this process. A short overview of the major ones are found below:

IGF-1:

IGF-1 comes in two varieties – paracrine IGF-1 is made primarily in the liver and autocrine IGF-1 is made locally in other cells. Paracrine IGF-1 travels through the bloodstream to the various tissues of the body, but autocrine IGF-1 is local in that in affects only tissues in the area in which it is released. Receptors on the surface of the cells are necessary for paracrine IGF-1 to enter the cells and exert its anabolic effects. But autocrine IGF-1, which is manufactured and released in the muscle cell as a response to high tension contractions, operates independently of receptors on the surface because it’s already inside. Once inside the cell, IGF-1 interacts with calcium-activated enzymes and sets off a process that results in protein synthesis (and the calcium ions that were released during muscle contraction and also the ones that leak into the muscle after the sarcolemma is damaged ensure that the necessary enzymes are calcium-activated).

A large part of this increase in protein synthesis rate is due to the fact that the IGF-1/calcium/enzyme complexes make protein synthesis at the ribosomes more efficient. By the way, insulin works at the ribosome in a similar manner, hence the name insulin-like growth factor-1 (IGF-1). So get some quick digesting carbs in after your workout to raise insulin levels.

GH:

GH is thought to work, primarily, by causing the cells (both liver and muscle cells) to release IGF-1. Effective training causes a rise in GH levels in the bloodstream; this GH prompts the liver to release paracrine IGF-1 several hours afterward, and also the muscle cells to release autocrine IGF-1, thus leading to another potential growth induction.

Prostaglandins:

Certain prostaglandins are released during contraction (and stretch); two of the most significant to growth being PGE2 and PGF2-alpha. PGE2 increases protein degradation, whereas PGF2-alpha increases protein synthesis.

But PGE2 isn’t all bad because it also powerfully induces satellite cell proliferation and infusion. The mechanism of PGF2-alpha’s action is much less clear but is suspected to be connected to increasing protein synthesis ‘efficiency’ at the ribosomes.

Testosterone:

‘Free’ testosterone (the kind that isn’t bound to a binding protein) travels freely across the muscle cell membrane and, once inside, activates what’s called the ‘androgen receptor’. ‘Bound’ testosterone (the kind that is bound to a binding protein) must first activate receptors on the cell surface before it can enter (the number of receptors on the surface is what controls this pathway). Once the androgen receptor is activated by testosterone it travels to the nucleus and sets off the protein synthesis process. In this way, testosterone directly causes protein synthesis and is, by far, the most powerful anabolic agent found naturally in the human body. Testosterone also increases the satellite cells’ sensitivity to IGF-1 and FGF, thereby promoting satellite cell proliferation and differentiation. It also increases the body’s systemic output of GH and IGF-1. Resistance training causes a spike in testosterone level.

After A Workout: To facilitate the growth process, muscle cells are more ‘receptive’ to testosterone, systemic IGF-1 and GH.

The whole process of cellular damage and subsequent overcompensation (the cells grow back a little bigger than they were before) can take anywhere in the neighbourhood of several hours to several days, depending on the severity and type of training. Trained individuals, however, have been shown, in several studies, to complete the protein synthesis cycle within 36-48 hours after intense ‘conventional’ Bodybuilding-type weight training. This is strong evidence to support the idea that muscles should be trained every 48 hours. Clearly, increasing the volume of muscular contractile elements is the key to increasing muscle size and strength. Since the type II fibers contain the most actin/myosin filaments, and generate the highest tensions, they have the greatest potential for strengthening/growth.

The prerequisite, of course, is that you have to lift weights heavy enough to recruit the type II fibers – and for them to twitch fast enough to develop significant tension. You also have to subject them to that tension long enough for significant damage to occur to the muscle fibers.

Still To Come

In Part II of this series I’ll present the reasons why you want to increase intracellular mitochondria number and discuss further why Weightlifters are, as a group, stronger than Bodybuilders but usually smaller. I’ll also wrap up with some very ‘innocent’, yet profound, recommendations regarding muscle growth and strengthening.